A rare earth sintered magnet is known as a magnet having the highest performance among permanent magnets. Conventionally, a rare earth sintered magnet has been widely used for a rotating device such as a servomotor or an elevator motor, a home appliance, and the like.
Particularly, for a rare earth sintered magnet, it is necessary to adapt to a high-temperature environment due to temperature increase when a rotating device is driven, and to prevent demagnetization due to a high temperature.
Therefore, a rare earth sintered magnet is required to have an excellent thermal tolerance and a high coercivity.
One of the most effective means for improving the coercivity of a rare earth sintered magnet is to use, as a material for a rare earth sintered magnet, an alloy having a composition in which light rare earth elements such as Nd are partially replaced with heavy rare earth elements such as Dy or Tb.
However, generally, a residual magnetic flux density of a rare earth sintered magnet contradicts a coercivity of a rare earth sintered magnet. Therefore, adding a heavy rare earth element such as Dy or Tb to a rare earth sintered magnet improves the coercivity of the rare earth sintered magnet. Meanwhile, the residual magnetic flux density of the rare earth sintered magnet is reduced. Therefore, it is difficult to achieve both of the coercivity and the residual magnetic flux density in the rare earth sintered magnet.
A heavy rare earth element such as Dy or Tb is a rare metal, and therefore the cost thereof is high. Therefore, it is desired to suppress the use amount of a heavy rare earth element such as Dy or Tb in terms of global environment conservation and magnet cost reduction.
In view of such a problem, in Patent Document 1, with regard to a permanent magnet used for a rotating device, a permanent magnet having a coercivity specially enhanced is coupled with a magnet having a low coercivity and a high residual magnetic flux density by an adhesive agent.
Thus, a permanent magnet having an enhanced coercivity can be used at a part where a temperature increases when a rotating electrical device is driven or where a demagnetization field from a stator is applied, and a permanent magnet having a high residual magnetic flux density can be used at the other part. Thus, a technique is disclosed in which coercivities of permanent magnets are selectively enhanced to achieve both of the residual magnetic flux density and the coercivity, thereby enabling reduction in the use amount of a heavy rare earth element.
In Patent Document 2, as a material for a permanent magnet, two kinds of material alloy powders of a first material alloy powder and a second material alloy powder are used which contain different amounts of heavy rare earth element. The first material alloy powder is a material alloy powder that contains no heavy rare earth element or contains a heavy rare earth element with a low content rate relative to the second material alloy powder. The second material alloy powder is a material alloy powder with a high content rate of heavy rare earth element relative to the first material alloy powder.
Further, a production method for R—Fe—B based sintered magnet is disclosed which includes a loading step of loading each of the first material alloy powder and the second material alloy powder into a predetermined space in a cavity formed by a mold, a step of obtaining a complex molded body composed of a first molded body portion made from the first material alloy powder and a second molded body portion made from the second material alloy powder, and a step of obtaining a sintered magnet or a permanent magnet having the first molded body portion and the second molded body portion coupled with each other by sintering the complex molded body.
In the loading step of this method, a partition in a vertical direction is provided in the cavity having a thin and long shape along the vertical direction, whereby the inside of the cavity is divided. Then, the first material alloy powder and the second material alloy powder are loaded into the divided cavities. Then, the partition in the cavity is removed, and the first material alloy powder and the second material alloy powder are pressurized and molded.
Thus, as in Patent Document 1, in the permanent magnet, the coercivity is selectively enhanced to achieve both of the residual magnetic flux density and the coercivity, thereby reducing the use amount of a heavy rare earth element.
In Patent Document 3, a material powder of an Nd2Fe14B compound alloy having a crystal grain composition, and a material powder of an alloy containing a heavy rare earth element such as Dy or Tb are separately produced, and then are mixed and sintered.
As a result of the sintering, the alloy containing the heavy rare earth element has a liquid phase. Then, the heavy rare earth element in the alloy is distributed so as to surround the Nd2Fe14B compound alloy. Therefore, in the permanent magnet, the coercivity can be increased while reduction in the residual magnetic flux density is suppressed.
In Patent Document 4, a fluoride containing a heavy rare earth element such as Dy or Tb is applied on a surface of a permanent magnet molded by sintering, and then thermal processing is conducted.
As a result of this thermal processing, the heavy rare earth element applied on the surface of the permanent magnet penetrates into the permanent magnet. Therefore, in the permanent magnet, the heavy rare earth element is present only at a desired portion.
Further, in the permanent magnet, by the thermal processing, the heavy rare earth element becomes denser only in the vicinity of an interface of a crystal grain, so that an anisotropy field in the vicinity of the interface of the crystal grain increases. Therefore, in the permanent magnet, the coercivity can be increased while reduction in the residual magnetic flux density is suppressed.
Thus, a production method for permanent magnet is disclosed which can achieve both of the residual magnetic flux density and the coercivity in a permanent magnet and reduce the use amount of a heavy rare earth element therein.